1. Field of the Disclosure
The present disclosure relates generally to scanners and scanning methods, and, more particularly, to methods used to determine the length of media placed on a flatbed scanner.
2. Description of the Related Art
Many scanning systems offer a variety of methods to automatically determine the size of a piece of paper being scanned. Automatic document feeders, for example, may use sensors to detect the width at which the paper guides are set, and also might use mechanical flags or optical sensors to determine when the top and bottom of the page pass through the scanner, thus determining the paper length. Sensing paper size on the flatbed can also be done using a variety of techniques, such as taking a quick scan at a fixed location on the flatbed as the flatbed cover is being closed. In this scan, the scan bar is positioned under the flatbed glass where the paper would be placed. Without the pressure pad backer yet present, the scanned area not covered by the paper will be significantly darker than the paper due to the scanner lid not being completely closed, and image processing can then be used to calculate paper length and width. Certain scanners actually require the user to leave the flatbed cover open when automatic paper size sensing is enabled so that a similar method may be employed to find the width or length of the paper. Another method employed on flatbed scanners involves a hardware infrared light emitting diode (IR LED) and paired photodiode that determine the reflectivity at a fixed point before scanning begins. This technique can be used to determine if paper is present or not at that fixed point to distinguish between two paper sizes. This can be useful on an A4 scanner to be able to distinguish whether the original is, for example, legal versus letter length, assuming the user has placed the paper in the correct position.
Each method has advantages and drawbacks. For cover-open scans, the advantages include the ability to determine both width and length of paper without the use of a hardware sensor. Disadvantages include poor image quality and susceptibility to ambient lighting artifacts. For fixed location scan as the cover is closing, the advantages include the capability to distinguish the width of the paper without the use of a hardware sensor. The disadvantages are paper length cannot be determined and a decrease in throughput because of the extra time required to move the scan bar to the top of page to begin scanning. Using an IR LED/photo diode sensor pair has the advantage of speed of detection but also has the disadvantages that neither width nor length can be determined except when the media is present at a single point on the scan bed and the cost to implement these sensors. The IR LED sensor is adequate where image quality is critical and the scan bar is not fast enough to move back to the top of the page for the scan without negatively impacting the user's experience but the cost of the IR LED sensor is a drawback.
Accordingly, it would be an advantage to have a device and method to provide the automatic size sensing without needing to add the cost of sensor hardware. It would be a further advantage to be able to eliminate pre-scans of the media sheet to determine media length. A still further advantage would be the ability to automatically determine media length while scanning the media sheet for image processing.